In the past, mobile communication systems have primarily used circuit-switched networks to provide voice services and low speed data services and packet-switched networks for high-speed data services. In circuit-switched networks, a dedicated channel is allocated for each voice or data call. In packet-switched networks, data is transmitted in packets over shared network resources. In general, packet-switched networks provide increased bandwidth efficiency as compared to circuit-switched network, while circuit-switched networks typically provide higher quality of service guarantees. In third generation (3G) packet-switched data networks have been integrated with circuit-switched voice networks to provide both voice and data services.
The fourth generation (4G) standard under development known as Long Term Evolution (LTE) is a packet-switched network and does not have inherent support for voice services. A number of proposals are under consideration for providing voice communications in LTE networks. However, it is uncertain at this point whether the initial roll-out of LTE systems will include support for voice communications. If support for voice communications is not available, the service providers can leverage existing circuit-switched networks to provide voice services. Even if the early LTE systems support voice communications, the service providers will likely phase in LTE systems gradually and leverage existing 3G networks to provide service in areas where LTE networks do not provide coverage. Therefore, interworking protocols are needed to enable interworking between LTE and existing circuit-switched networks.
Several proposals are being considered to enable interworking between 3G and 4G networks to allow service providers to leverage existing networks and gradually phase in LTE networks. One approach to interworking is known as Single Radio Voice Call Continuity (SRVCC). The SRVCC approach allows a LTE voice call to be handed over to a 3G network when LTE coverage is not available. The SRVCC approach is described in 3GPP TS.23.216. Another interworking approach is known as Circuit-Switched Fallback (CSFB). CSFB is an interworking mechanism that allows service providers to use existing circuit-switched networks to provide voice services to LTE users. A mobile user can register with the circuit-switched network after attaching to the LTE network. For voice communications, the user is redirected from the LTE network to a legacy network providing voice services.
To implement interworking protocols, an interworking function will be added to existing circuit-switched networks to enable circuit services domain messages to be sent to and from mobile terminals operating in the LTE network. To implement the interworking function, a mechanism is needed to specify the interworking protocol.